If it is not a fitting issue it is an issue of disrespect and lack of training! QH of course intend to buck, but you should ground train them and work the buck out of them. Whenever he bucks make him work, on the ground or under the saddle, the last thing you want is to get scared and make him go back to a walk! That's exactly what he wants! So if you are not comfortable in riding the bucks out of him then make him work from the ground, if he bucks he has to work, if he is good reward him! What is the horses age btw.